/Engine/Source/Runtime/Engine/Classes/Engine/Polys.h
Split with plane. Meant to be numerically stable.
int32 SplitWithPlane ( const FVector3f & InBase, const FVector3f & InNormal, [FPoly](API\Runtime\Engine\Engine\FPoly) * FrontPoly, [FPoly](API\Runtime\Engine\Engine\FPoly) * BackPoly, int32 VeryPrecise ) const